Rhodium alloy



United States Patent C 2,992,099 RHODIUM ALLOY Frank Herbert Reid, Houuslow, England, assignor to The International Nickel Company, Inc., New York, N.Y.,

a corporation of Delaware No Drawing. Filed Aug. 1, 1958, Ser. No. 752,421 Claims priority, application Great Britain May 10, 1956 2 Claims. (Cl. 75-172) The present invention relates to the electrodeposition of rhodium and, more particularly, to the deposition of crack-free rhodium electrodeposits.

Electrodeposits of rhodium both for decorative and industrial purposes are well known. The so-called decorative deposits are used, for example, in the protection of silverware and base metals and also in scientific apparatus. Industrial deposits, which are thicker than decorative deposits and are resistant to corrosion and to mechanical wear, are used, inter alia, as the surfaces of sliding electrical contacts. The thickness of a decorative deposit is generally between 0.000005 and 0.00005 inch, and that of an industrial deposit is generally between 0.0001 and 0.002 inch or greater. The electrolytes commonly used are solutions of rhodium sulphate containing various concentrations of sulphuric acid; but other acid solutions, for example, of rhodium phosphate or fluoborate, can be used. A substantially unidirectional electric current is passed through these baths and rhodium is deposited at the cathode.

Rhodium deposits as produced from the prior art baths show a pronounced internal tensile stress, which is dependent to some extent on the deposition conditions, but is generally of a high order, e.g., 50 to 100 tons/m In deposits thicker than about 0.0001 inch, the stress often leads to cracking of the rhodium deposit in the as plated condition. Cracks formed in this way may be referred to as primary cracks and usually extend right through the deposit to the basis metal. They may be detected by the so-called electrographic method, in which the basis metal, which is exposed by the cracking, is made toproduce a pattern on a suitable test paper. Cracks are of course undesirable, particularly when the object upon which rhodium is deposited is exposed to a corrosive environment and my object is to reduce or eliminate this cracking tendency.

Although many attempts were made to overcome the foregoing difficulties and other disadvantages, none, as far as I am aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that by the use of novel rhodium electroplating baths, substantially crack-free electrodeposits may be produced under normal commercial electroplating conditions.

It is an object of the present invention to provide a novel electrochemical plating bath which enables one to reduce or substantially eliminate cracks in rhodium electrodeposits.

A further object of the present invention is to provide a novel alloy having particular advantages as an electrodeposit.

A still further object of the present invention is to provide a novel electrodeposited alloy characterized by excellent resistance to tarnishing and corrosion and freedom from cracks in the as-plated condition.

Other objects and advantages will become apparent from the following description.

Generally speaking, the present invention contemplates the production of electrodeposits of rhodium through the use of an aqueous, acidic electrodeposition bath which contains a soluble rhodium salt and at least sufficient free mineral acid to prevent hydrolysis of the selected rhodium salt and selenium in an amount equivalent to about 0.05 gram per liter (g.p.l.) to about 25 g.p.l. of selenic acid. It is preferred to add at least about 0.05 g.p.l. of selenic acid and/or of one or more salts of said acid to the rhodium plating bath in order to provide an amount of selenate ion therein which is effective in reducing the primary cracking tendency of the deposit. The soluble rhodium salt suflicient to provide from about 5 to 20 grams per liter of dissolved rhodium, for example, about 10 g.p.l., may be selected from the group consisting of rhodium sulphate, rhodium phosphate and rhodium fluoborate. The free acid may be selected from the group consisting of sulphuric acid, phosphoric acid and fluoboric acid and may be employed in amounts at least up to about milliliters per liter (m.p.l.). The necessary concentration of selenic acid is supplied either by the addition of the acid itself or by the addition of a salt which upon hydrolysis in the presence of a mineral acid will produce a satisfactory effective quantity of selenic acid. Of course, when adding the hydrolyzable salt, it is preferred that the reaction products of the hydrolysis reaction'will not interfere with the electrodeposition. Accordingly, it is preferred to use a sodium or other alkali metal salt of selenic acid when a salt addition is indicated.

In carrying the invention into practice, it is preferred to provide a range between a minimum of at least 0.05 g.p.l. to a maximum of about 1 g.p.l. (or even a maximum of about 8 g.p.l.) of selenic acid in the aqueous acidic rhodium electroplating bath. I prefer to add free selenic acid. Although any addition of selenic acid will reduce the cracking to some extent, I prefer always to add at least 0.05 g.p.l. The amount necessary to ensure practical freedom from cracking under all conditions of deposition is about 0.4 g.p.l. in an electrolyte low in free mineral acid, e.g., sulphuric acid, and rather less in an electrolyte with a higher concentration of free mineral acid, e.g., sulphuric acid. No advantage is gained by adding more than 8 grams of selenic acid per liter. Rhodium sulphate is the preferred source of dissolved rhodium and a preferred electrolyte according to the invention may thus have the following composition:

Rhodium (as sulphate) gms./liter 5 to 20 Free sulphuric acid mls./liter Up to 100 Selenic acid gms./liter 0.05 to 0.8

The temperature of operation and the current density are not affected by the addition of selenic acid. The temperature may be from about 20 C. to about 70 C., preferably above 50 C., and the current density from about 0.5 to about 2 amperes per square decirneter (amps./ dm. Especially advantageous results are obtained when the current density is about 1 amp./dm. Rhodium sulphate electrolytes are commonly made by dilution of concentrated solutions, themselves made by dissolving rhodium hydroxide in sulphuric acid. As a general rule, therefore, the electrolyte contains a small amount of free sulphuric acid. Further sulphuric acid is often added to increase the content of free sulphuric acid.

While it is to be observed that the present invention contemplates the minimization of primary cracking tendency in rhodium electrodeposits produced over a broad range of operating conditions, certain relationships between operating conditions and bath composition should be maintained in order to assure the best results. For example, if the temperature is maintained on the low side of the aforementioned range, the concentration of free mineral acid should be increased. From the standpoint of smoothness of deposit, an operating temperature of about 50 C. has been found to be most effective. Using the same criterion, a current density of about 1 amp./dm. has been found to provide optimum results.

Patented July 11, 1961- Again it is to be noted that as the concentration of selenic acid is increased, the hardness of the rhodium deposit is decreased due to an increase in selenium content in the deposit. Thus, one should introduce a maximum of about 1 g.p.l. of selenic acid into the bath where hardness of the deposit is an important characteristic of the deposit. As was indicated hereinbefore, to achieve optimum minimization of primary cracking tendency, the amount of selenic acid added to the electrolyte should be increased as the amount of free mineral acid is decreased. As a general rule, it is preferred to add a minimum of 0.4 g.p.l. of selenic acid to the electrolyte regardless of the free mineral acid concentration.

It is to be observed that the present invention contemplates a more advantageous rhodium electrodeposition bath which contains ingredients in the folowing tabulated ranges of concentration in units of g.p.l.

1 Milliliters per liter.

Likewise, the operating conditions are tabulated in Table II:

TABLE 11 Conditions Broad Desired Temperature 20 C.70 C 50 C. Current Density 0.5-2 amp/rim 1 ampJdmJ It is to be observed that while the specification has disclosed that the electrodeposits produced from the aforementioned baths of the present invention are of metallic rhodium, they contain small amounts of selenium, the amount thereof depending to a certain extent on the concentration of selenic acid used in the bath. For purposes of this specification, it is intended that the terms rhodium or electrodeposited rhodium" when used in reference to the electrodeposits include these small amounts of selenium. In view of the inclusion or codeposition of selenium with rhodium, it should be noted that when the baths disclosed herein are replenished, not only rhodium must be added thereto but also suitable additions of selenic acid should be made.

Rhodium-selenium alloys deposited from baths as set forth in Table I will contain from small amounts, e.g. 2% up to about 50% by weight of selenium and still be free from the type of cracking which is revealed by electrographic testing. The selenium content of the deposits produced with the preferred additions of selenic acid (not exceeding 1 gram per liter) is of the order of about 2% by weight, e.g., from about 0.05% to about 2%. The balance of the alloy is, of course, essentially rhodium.

For the purpose of giving those skilled in the art a better understanding of the invention, the following illustrative examples are given:

Example I from this electrolyte at a temperature of 45 C. with a v current density of 2 amp./dm. showed severe cracking.

After adding selenic acid to the electrolyte to give a concentration of 0.4 gm./liter, an uncracked deposit 0.001 inch thick was obtained under the same operating conditions.

Example II Electrolyte composition:

Rhodium (as sulphate) gms./liter 10 Added sulphuric acid mls./liter 70 A deposit 0.0007 inch thick obtained from this electrolyte at a temperature of C. with a current density of 2 amp./drn. showed moderate cracking. After addition of selenic acid to give a concentration of 0.4 gm./liter, an uncracked deposit 0.0012 inch thick was obtained.

Example III U-shaped specimens of copper rod were plated in an electrolyte of the composition given in Example II both before and after the addition of selenic acid to give a concentration of 0.4 gm./liter. The thickness of rhodium in each case was 0.0005 inch. The specimens were then immersed in concentrated nitric acid. After 25 minutes the specimen plated before the addition of selenic acid had completely dissolved, whereas the other specimen was unaffected.

Example IV :Flat copper specimens were plated on one side only with rhodium deposits 0.0005 inch thick from the electrolytes of Example III. The copper was then dissolved away by treatment with nitric acid. The deposit from the electrolyte that was free from selenic acid disintegrated into numerous fine flakes because of cracks in it, but the deposit from the treated electrolyte remained as a coherent foil.

The present invention is particularly applicable to the electrodeposition of rhodium on any metal which is compatible with the acid electrolytes. These metals include nickel, nickel-silver, silver, gold, platinum, palladium, copper and brass.

It is to be observed that the present invention provides a process for the production of deposits of rhodium having reduced cracking tendencies from an acid electrolyte and also provides a novel acidic electrolyte for the production of such rhodium electrodeposits.

The present application is a continuation-in-part of applicants copending prior application Serial No. 656,499, filed May 2, 1957, now Patent No. 2,866,740.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are con sidered to be within the purview and scope of the invention and appended claims.

I claim:

1. A selenium-rhodium alloy which comprises from a small amount up to about selenium with the balance essentially rhodium,

2. A selenium-rhodium alloy which comprises between about 0.05% and about 2% selenium with the balance essentially rhodium.

References Cited in the file of this patent UNITED STATES PATENTS 2,384,501 Streicher Sept. 11, 1945 2,866,740 Reid Dec. 30, 1958 FOREIGN PATENTS 215,532 Australia June 18, 1958 

2. A SELENIUM-RHODIUM ALLOY WHICH COMPRISES BETWEEN ABOUT 0.05% AND ABOUT 2% SELENIUM WITH THE BALANCE ESSENTIALLY RHODIUM. 